The present invention relates generally to generating sputtering to produce a coating on a substrate, more particularly, to high power impulse magnetron sputtering (HIPIMS).
Sputtering is a physical process whereby atoms in a solid target material are ejected into the gas phase due to bombardment of the material by energetic ions. The process of sputtering is commonly used for thin-film deposition. The energetic ions for the sputtering process are supplied by a plasma that is induced in the sputtering equipment. In practice, a variety of techniques are used to modify the plasma properties, especially ion density, to achieve the optimum sputtering conditions. Some of the techniques that are used to modify the plasma properties include the usage of RF (radio frequency) alternating current, an AC power source, a DC power sources, a superposition of DC and AC power sources, a pulsed DC power source such as a bipolar or unipolar power source, the utilization of magnetic fields, and the application of a bias voltage to the target.
Sputtering sources are usually magnetrons that utilize magnetic fields to trap electrons in a closed plasma loop close to the surface of a target. The electrons follow helical paths in a loop around the magnetic field lines. The electrons undergo more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. The sputter gas is inert, typically argon, though other gases can be used. The extra argon ions created as a result of these collisions leads to a relatively higher deposition rate. It is known to arrange strong permanent magnets beyond the target in order to create such a magnetic field loop. At the location of the plasma loop on the surface of the target, a racetrack can be formed, which is the area of preferred erosion of material. In order to increase material utilization, movable magnetic arrangements are being used, that allow for sweeping the plasma loop over relatively larger areas of the target.
Direct current (DC) magnetron sputtering is a well-known technique using crossed electric and magnetic fields. An enhancement of DC magnetron sputtering is pulsed DC. The technique uses a so-called “chopper,” where an inductor coil L and a switch are used to modify a DC power supply into a supply of unipolar or bipolar pulses, see FIG. 1. The inductor coil L is the chopper and can preferably include a tap located between the DC power supply and the magnetron cathode. The electronic switch S is periodically open and closed to create the pulses. In the on-time of the switch S, an effective shortcut between the tap of the coil L and the magnetron anode switches the negative cathode voltage off, preferably overshooting to positive voltages by the auto transforming effect of coil L. During off-time, the current from the DC power supply continues to flow into the coil L and storing energy in its magnetic field. When the switch S is again off, a short negative high voltage peak is formed at the magnetron cathode. This helps for relatively fast reigniting of the magnetron plasma and restoring the original discharge current.
The High Power Impulse Magnetron Sputtering (HIPIMS) technology as described in the prior art uses relatively lower repetition frequency of pulses typically 5 Hz to 200 Hz, and pulse times 20 to 500 μs. The discharge peak currents range from 100 A for relatively small cathodes up to 4 kA for relatively large cathodes, which corresponds to current density at cathode in the order of magnitude of 0.1 to 10 A/cm2. A common technique uses wiring as in FIG. 2.
In FIG. 2 and FIG. 3, the DC power supply charges a bank of capacitors C to a starting voltage, that is discharged into the magnetron through a cable with some inductance Lcab and resistance Rcab. Optionally, an inductance L is introduced in series to limit the rise time of the magnetron discharge current. In FIG. 3, an arc is detected by an amp meter. When an arc is occurring during a pulse at the power supply, the capacitor is disconnected and only the energy left in the cable plus optional coil L1 is discharged into the arc discharge.
FIG. 6 shows the result of an experiment. The data shows the measured rise time of the current as a function of frequency in a state of the art HIPIMS discharge. The target in this example was made of Tantalum (Ta), with the target having a diameter of 300 mm, and the experiment was using a rotating magnet array. For low repetition frequency of 10 Hz (100 ms period), there is a relatively long delay (about 5 μs) between the start of the voltage pulse and the start of the current rise. The delay is somewhat shorter (over 4 μs) when a repetition frequency of 100 Hz (10 ms period) is used. With a relatively higher frequency of 500 Hz (2 ms period), the current starts to rise much faster, within only about 1.5 μs.
There are a number of disadvantages with the standard HIPIMS technique. One disadvantage is that relatively large and expensive capacitors dimensioned for high stored energy and for substantially voltage other than the magnetron operation voltage are provided. Another disadvantage is that the magnetron is operated in a mode close to constant voltage, in contrast to operating the magnetron in a constant current mode. One other disadvantage is that there is a long starting time of the magnetron current pulse (5 to 100 μs), as seen in the experiment of FIG. 6. Another disadvantage is that there is a long delay between the voltage pulse and the start of the current rise (2 to 20 μs).
Using a chopper is a good choice when relatively high duty cycles (50% to 99% on-time), short off-times (100 ns-10 μs) and high frequencies (10-500 kHz) are used. A duty cycle is equivalent to the percentage of an on-time divided by the cycle time. Under such circumstances, the energy losses in the coil stay acceptable and the coil size is not too large. However, HIPIMS uses relatively low frequencies (5 Hz to 200 Hz), and low duty cycles of 0.01% to 10%. This is not favorable for chopper operation, as a full peak discharge current should flow through the coil over the long plasma off period, causing high resistive losses in the coil. Also the coil dimensions for the enormous stored energy are not practical.
The prior art literature of Helmersson, Christie, and Vl{hacek over (c)}ek show that one of the major drawbacks of HIPIMS is a relatively lower deposition rate per average input power. The reason for the lower deposition rate is that the sputtered material can be relatively highly ionized in a well developed HIPIMS discharge and the ions are attracted back to the cathode and therefore most of them do not reach the substrate.
The high discharge current values used in HIPIMS also create an increased risk of arcing. It is well known that when an arc occurs, the discharge voltage drops down to the range 10-100V, the current increases, and the discharge itself contracts to one or more tiny hot cathode spots. Arcs are undesirable because the energy in the system discharges too rapidly. Damage done by such hot spots to the surface of the target has been described as well as emission of droplets and particles. Some power supplies that have been used for HIPIMS use arc suppression capability. A common technique for arc suppression uses the wiring scheme of FIG. 3.
The occurrence of arcing is shown by the plots in FIG. 4 and FIG. 5. FIG. 4 shows a typical HIPIMS pulse without arc occurrence, whereas FIG. 5 is an example of an arc event. When the arc occurs, the discharge impedance decreases drastically. This results in a sharp increase of current, but the increase is limited in amplitude and rise by the serial impedance of Lcab, L1 and Rcab. When the arc is detected by one of the well-known techniques, e.g. by exceeding a threshold current, the switch S1 is immediately set to an off-state, only with a delay limited by the used electronics and the switching element. Nevertheless, the energy E stored in the cable and the coil may be relatively large. If the peak current before switch-off is Ipeak and effective inductance L=Leab+L1, the energy E=L Ipeak2/2. As an example, Ipeak=2 kA and L=1.0 μH gives energy E=2 J. This is an energy that is able to deliver for example arc current of 200 A at arc voltage 50V for a period as long as 200 μs.
In technological processes using HIPIMS for deposition of decorative or hard coating the standard arc suppression described above may be enough to prevent or limit target damage and enable reasonable quality deposition, especially when pulsing conditions are carefully chosen to prevent or limit frequent arcing and to inhibit the formation of arcs. Nevertheless, for processes sensitive to particles, such as semiconductor wafer processing, thin film heads, MEMS, and optical or magnetic data storage, the residual energy fed into an arc may be too large. Droplets emitted from arcs during deposition may cause malfunction of some devices being produced on a silicon wafer or another substrate. An even worse problem can be the accumulation of particles in the processing chamber, for example after installing a new target. Such new targets often exhibit a much larger tendency to arcing than older, sputter-eroded targets. Particles accumulated in this way may be released from the reactor walls later during production processes, even if arcing during the process itself does not occur.
The standard techniques used for arc suppression have a disadvantage of leaving too much energy available for an arc discharge after the arc suppression electronics reacted. This energy in the order of magnitude of 0.1 to 10 Joule is enough for heating up the cathode spot, developing a full arc discharge and emitting droplets on most technically interesting target materials. It is desirable to limit the arc energy and lifetime of the arc after it is detected.